Group notification

hello my website is very involed in the groups , its the most important point of the website
so the problem is with the notifications , they are a bit useless with them and my website idea would never work without notifications , what i want is to make users who joined a group to get notified when any admin of group make a new post in the group .
i want them to be notified in the website not by email, cause by doing some scearch all i can find is email notification and im not looking for those.
thanks in advance